The present invention concerns an apparatus for producing a water seal in a protective tube of a submersed pump of a gas cistern with the aid of which apparatus the pump can be safely serviced or replaced without the risk that liquefied gas would enter the protective tube.
Liquefied gases may, as is known in the art, be stored in rock cisterns in which the leakage water from the rock is gathered on the bottom of said cistern to form a water bed. With the aid of the water bed, also the surface height of the liquefied gas can be regulated in the cistern.
Liquefied gases are generally removed from the cistern with the aid of a submersed pump placed in a protective tube. The protective tube makes it feasible that the submersed pump can, if needed, be detached for servicing or replacement. However, this operation prerequires that the protective tube can be emptied from gas and that the gas entry into the protective tube can be prevented during a servicing measure.
A problem solution for replacing and maintaining submersed pumps is to provide a water seal on the lower end of the protective tube, whereby penetration of gas into the protective tube is prevented. In order to produce a water seal, the protective tube may form a U-shaped bend in that the tube incoming end is on a higher level than the lowermost point of the tube and that it extends to the liquefied gas layer on a plane which within the cistern is higher than the surface of the water bed. The provision of the water seal starts so that pressurized nitrogen gas is conducted into the protective tube to the extent that the surface of the liquefied gas within the protective tube falls to the adjacency of the bend located in the lower end of the protective tube. Thereafter, water is added into the protective tube, whereby a water seal is produced in the bend of the protective tube. After that, pressurized nitrogen gas is discharged from the protective tube and, at the same time, an equivalent quantity of water is added into the protective tube, whereby the water seal is preserved in place.
A problem related to the production of a water seal of this type is that an appropriate quantity of water must be added into the protective tube per discharging nitrogen gas unit. However, measuring such gas quantity under field conditions is a cumbersome task. If too little water is added therein, there is a risk that the water seal breaks and liquefied gas flows into the protective tube. This creates a great hazard in an instance in which the breaking of the water seal occurs while the upper end of the protective tube is open when replacing the pump. If too much water is added therein, it will flow through the protective tube into the rock cistern without being observed at all. The excessive quantity of water may cause difficulties in the operation of the rest of the pumps if the surface of the water rises too high.